With the increasing awareness of the negative impact of pollutants and greenhouse gases particularly the gas components SOx, NOx and CO2 on the environment, many emission control legislations are now in place to control the emission of these pollutants and Greenhouse gases. Numerous attempts have been made to meet the requirement of these legislations, among which one way is to go for post combustion gas abatement technology. However, this technology is not satisfactory due to the complex nature of the flue gases. It has been known that the composition of the flue gases differ greatly and depends on the type of the fuel used. However SOx, NOx and CO2 are three main common gases produced simultaneously in the combustion process. Hence, it is impractical to consider abatement of one single gas only while ignoring the conflicting influence from other gases.
Most of the technologies available in the art are directed to removal of a single gas, which will be described below.
SO2 is usually removed by wet or dry lime scrubbing process. This is either a liquid or solid phase chemical reaction process for removing SO2 only but not for removing NOx. In fact, production of the lime CaO would lead to emission of CO2, when the raw material CaCO3 is heated to obtain the CaO. Lime is used as a consumable reactant in the process.
SO2 can also be removed by seawater or freshwater scrubbing with or without the addition of NaOH. In this process, the SO2 is removed by liquid phase chemical reaction with the concurrent generation of environmentally harmful acidic water or CO2. Alkalinity in the water or the added NaOH is used as consumable reactant in the process. Again, this process is not capable of removing NOx or CO2.
NOx is commonly removed by a Selective Catalytic Reduction (SCR) process. In the SCR process, urea or ammonia is used to reduce the NOx into nitrogen and oxygen. The SCR process is capable of removing the NOx only but cannot remove SOx or CO2. In fact, SOx will “poison” the catalyst in the SCR process. Another disadvantage of the SCR process is that high temperature over 300° C. is required for catalyzing the gas reduction. This is impractical in the post combustion systems of many combustion plants, especially when installation of waste heat recovery is needed to recover residual heat from the exhaust gas. In the SCR system, a liquid phase chemical reaction which utilizes high thermal energy to perform the catalytic gas reduction takes place.
NOx can also be removed by chemical oxidation processes, in which NOx is forced to react with oxidizing agent such as ozone and acids to convert NO to NO2 and eventually to N2O5 before it is dissolved in the water to form nitrate or nitrite. This process may cause an environmental issue because of the formation of the high content of nitrate or nitrite in the discharge water. This process is not capable of removing CO2 and SOx.
Generally, CO2 in the flue gas is hundreds times higher than SOx or NOx in volumetric composition. There is no suitable or viable technology available in the art to remove CO2. One approach to deal with CO2 is the so-called Carbon Capture and Storage (CCS) method which utilizes chemicals, for example monoethanolamine (MEA), to capture the CO2 and release it back for storage by heating. This method is to underground store the CO2 instead of letting it going into the atmosphere.
In general, the available technologies in the art for the abatement of SOx, NOx and CO2 gases in a large scale are involved with the use of chemicals and are implemented via liquid phase reactions.
An electro-chemical process is proposed to produce sodium hydroxide for removal of the pollutant gases (Sukheon An and Osami Nishida, JSME International Journal, series B, vol 46, No 1, 2003). Essentially, the process is using the electrolyzed seawater to produce the chemical NaOH to perform the liquid phase neutralization and thus to remove the gases SOx and CO2. The acid water produced at the anode side is then used to oxidize the NOx into nitrate via liquid phase chemical reactions. As discussed in this article, the entire process is working on the principle of liquid phase chemical reactions. SO2 is converted into sodium sulfate in liquid phase reaction, NOx is converted into sodium nitrate in liquid phase, and CO2 is converted into bicarbonates and carbonates in liquid phase. This process has a discharge issue of nitrate and requires a mole equivalent weight of NaOH to carry out the liquid phase reaction in order to remove each gram of CO2. This will inevitably involve an immense amount of NaOH or a large amount of electricity to electrolytically produce the NaOH. An accompanying problem of using this process is that the amount of CO2 generated during the electrolysis process is likely more than the amount of CO2 that is needed to be removed from the flue gases. In addition, the nitrate resulting from the removal of the NOx and the wash water produced in this process will cause an environmental problem unless an extensive waste water treatment plant is separately installed to treat the waste water. If the process is implemented in a closed loop system, the system will reach a saturation point where no further liquid phase reaction takes place when the limited amount of reactants in the seawater or freshwater is used up for conversion of CO2, SOx and NOx. This limitation makes it impractical to use such a process in closed loop applications. This process may find an application in an open loop system only, since the supply of seawater reactant is continuous. However, the issues of the produced nitrate, waste water and the problem of the energy required to remove the CO2 remains unresolved.
All the above methods are based on liquid phase reactions. Presently, there are various methods and systems which use a dry plasma technique to remove the gas NOx. The common feature of these methods and systems is to use a reaction chamber with high voltage to create the plasma. The generation of the plasma normally requires a clean environment which is practically impossible to achieve in coal or fuel combustion flue gas systems. In the dry plasma-based methods, electron streams are generated by high voltage and high power to bombard the gas molecule NOx so that NOx is broken into N2 and O2, as discussed in U.S. Pat. Nos. 7,240,484, 7,377,101, 7,198,764, 7,188,469.
The drawbacks of using the dry plasma-based methods and systems are that a clean environment is required and that high voltage and high energy are consumed. Especially, the energy consumed for the removal of CO2 will be very high. This results in limited applications of the dry plasma-based methods and systems in a large scale, especially when the abatement of all the gases SOx, NOx and CO2 is necessary.
As can be seen, the pollutants and greenhouse gases SO2, NOx and CO2 are all produced and mixed together in the flue gas, and all are needed to be removed before the flue gas is emitted. However, the above processes and systems are either intended for removing one of the pollutants and greenhouse gases, or are not a practical solution. If all these gases are to be removed together by the respective processes, installation of three different treatment plants will be necessary for the removal of all three gases. This inevitably results in high capital cost, large storage space, and high cost of reagents, in additional to the issues of storage and disposal of final products.
There is a need for a method and a system that are capable of simultaneously removing the pollutants and greenhouse gases from a flue gas at low cost, and do not cause a harm to the atmosphere and marine environments without the need of consideration of disposal of final products and storage of raw reagents.